Flexible semiconductor devices offer several advantages over more conventional wafer based semiconductor devices for mobile devices, wearable devices, and lower cost devices. Flexibility is usually defined by the radius of curvature in kilometers that the flexible semiconductor device can be flexed to. In general, flexible semiconductor devices in the prior art are formed in a wafer state and then either removed from a growth substrate, thinned, or otherwise separated from a rigid element. While this allows for processing using standard semiconductor wafer based lithography or patterning equipment tool sets, a critical benefit of flexibility is lost using this approach. This invention discloses flexible semiconductor devices grown on flexible epitaxial elements whereby improved device performance is realized from the flexible stress free nature of the growth substrate or epitaxial element and the presence of a virgin as grown epitaxy ready surface.
Nitrides and zinc oxides and their alloys in particular offer a unique range of properties compared to other semiconducting materials like silicon and silicon carbide. Wide band gap materials like gallium nitride (GaN), aluminum gallium nitride (AlGaN), and aluminum nitride (AlN) are finding applications in high frequency devices, high power devices, LEDs, optoelectronics, bio-technology, and high efficiency electronics. The opportunity exists for integration of these devices into single layered and multi-layered devices. Nitrides and zinc oxides and their alloys offer a unique mix of optical transparency through the visible and UV spectrum, high thermal conductivity, chemical resistance, piezoelectric properties, laseability, bio-compatibility and high frequency capability. Like nitrides and zinc oxides, diamond and other high bandgap materials are difficult to make into wafer form. By harvesting epitaxial elements of these high bandgap materials the benefits of bulk wafers can be realized at a fraction of the cost and without the need for expensive and defect prone steps like slicing and polishing.
The use of nitrides and zinc oxides and their alloys in most applications has been limited by the lack of low cost high crystal quality native substrate material. Nitrides and zinc oxides and their alloys have proven difficult to grow economically in high quality single crystal form due to the high temperatures and narrow growth conditions. While this work may eventually yield low defect density material, the cost of those materials will be inherently high.
Forrest in U.S. Patent Publications No. 2011016910, “Methods of preparing flexible photovoltaic devices using epitaxial liftoff, and preserving the integrity of growth substrates used in epitaxial growth” discloses methods to remove epitaxial devices to form flexible solar cells via liftoff. Like most flexible approaches this disclosure removes the epitaxial layer after device growth. The epitaxial stresses are higher than when the devices are formed, grown, or otherwise deposited directly on a flexible growth substrate. In addition, the device performance is typically compromised when the growth substrate is removed or at least different. As an example, the emission wavelength of an MQW LED is typically different on the growth substrate than when it is transferred via wafer bonding to another device substrate. This can lead to increased binning requirements and yield issues.
Lee in U.S. Patent Publications No. 2008 0248259 “Gallium Nitride/Sapphire thin epitaxial element having reduced bending deformation” discloses a method for creating a GaN on sapphire growth substrate with a reduced radius of curvature. The goal of Lees filing was to increase the radius of curvature to 1 kilometer or higher. In general flat wafers are preferred for uniformity and subsequent lithographic and processing steps. However, the radius of curvature changes for any non-homogenous material as a function of temperature due to the mismatch in the thermal expansion coefficients. A flat growth substrate at room temperature may have a radius of curvature less than 1 kilometer at 1000° C. growth temperatures or vice versa. So, while a low stress layer may be grown at the growth temperature, once cooled to room temperature very high stresses may be imparted to the layer. Even a native growth substrate will only exactly match those layers which match the growth substrates exact composition. All wide band gap materials require high temperature growth processes so there may be over 1000° C. difference between growth temperature and operational temperature for a device. The authors of this filing have shown that flexible freestanding single crystal epitaxial elements can reduce epitaxial stresses because they are flexible and can deform to alleviate stresses. They also can be forced flat for post processing steps like lithography etc. As shown in Lee slight changes in the radius of curvature can translate into very large changes in stress levels. The need exist for methods to eliminate stress due to thermal expansion mismatch.
Romano in U.S. Patent Application 20130143917 discloses the growth of InGaN layers for JFET's. It is claimed that 104 to 105 dislocations per cm2 are required to grow InGaN with greater than 20% Indium content. Bulk GaN with such low dislocation density requires the use of thick GaN growth (greater than 1 mm) which is both expensive and difficult to produce. The need exists for a low cost method of obtaining high quality InGaN with greater than 20% indium content without having to resort to thick bulk GaN wafers.
Zimmerman in U.S. Pat. No. 8,017,415 discloses methods for forming devices on the freestanding nitride foils disclosed in this filing on both sides of the foils. These foils are typically 30 to 50 microns thick harvested from sapphire templates using the technique disclosed in Zimmerman U.S. Pat. No. 7,727,790. Unlike conventional laser liftoff this approach allows for removal of large area foils without the need to touch or otherwise modify the as grown virgin epitaxial ready surface of the foil. This overcomes the surface, polish, and miscut angle defects associated with bulk GaN wafers. The intent of this invention is to disclose flexible semiconductor devices formed based on flexible freestanding nitride and zinc oxide and their alloys.
It is critical that any growth substrate exhibit an epitaxy ready surface for there to be quality additional epitaxial layer growth. Silicon has dominated the semiconductor market due to the ease of forming high quality wafers with epitaxial quality surfaces with a uniform crystal plane across the entire wafer. Unlike silicon, nitrides do not easily polish and are very difficult to grow such that a single crystal plane is presented across the entire growth surface within any one plane. The warpage and stresses experienced during bulk crystal growth means that when the bulk material is sliced into wafers the crystal orientation varies across any planar surface. Growth rates of subsequent epitaxial layers are directly related to crystal plane orientation. The need therefore exists for a low cost nitride and zinc oxide, and their alloys, growth substrate in the form of foils, tapes, or ribbons which has an as grown epitaxy ready surface for subsequent device growth.
SiC, Silicon, Glass, Sapphire, and a variety of other growth substrates have been used to grow nitride devices. Each has some drawback compared to native substrates. Of the list above SiC is probably one of the better thermal matches but is limited to less than 6 inches in diameter and is expensive to manufacture. In addition, doped SiC is optically absorptive in the blue region making doped SiC unsuitable for vertical blue LED devices. As such undoped SiC is used which then requires lateral devices which suffer from current spreading and higher Vf (forward voltage) performance. Silicon is reactive with nitrides and Si is the major n type dopant. Care must be taken to prevent poisoning of p type material. Glass requires lower growth temperatures and suffers from many other issues. Sapphire is the dominate growth substrate in the LED industry. It is however a dielectric and therefore most devices are lateral devices or the nitride must be removed to create a vertical device. Significant stresses are introduced into the nitride layer when sapphire is the growth substrate. The need exists for methods and substrates which mitigate the stresses from subsequent epitaxial growths.
Semiconductor devices are used in a wide variety of applications. Silicon has been used as the substrate material of choice due to cost and availability of single crystal silicon wafers. Driven by the microelectronic industry, silicon wafer production has enabled the economical use of larger wafers with greater than 12 inch diameters. The low cost of silicon allows thick (1-2 mm) single crystal silicon wafers to be used without a secondary substrate for handling and processing. This enables the formation of large area die, which can be processed without the need for transfer substrates or wafer bonding. Additionally, the properties of silicon permit wafers to be doped so that 3 dimensional devices (via planar processing) can be created taking advantage of the wafer conductivity. For some devices it is desirable to utilize thinning techniques to reduce the overall thickness of the silicon to tens of microns to improve thermal performance. Although silicon has been the dominant material for most microelectronic applications there are other materials, which have desirable properties and advantages over silicon in the areas of optoelectronics, solar cells and power devices. However, heretofore there has not been an economical solution to using these materials without growing them or attaching them to secondary substrates.
Silicon also has limitations with regard to operating temperature as well as other critical material parameters; this limits silicon's usefulness in optical, power and high-frequency applications. Nitride alloys and oxide alloys have several properties, which are superior to silicon ranging from higher thermal conductivity to biocompatibility. Unfortunately, nitrides are not available in wafer form at a reasonable price or reasonable quality. Even if such wafers were to become available, costly growth, dicing, and thinning techniques would be required to create useful devices. In most cases, devices with an overall thickness between 20 and 150 microns are desired from a thermal impedance, packaging, and optical efficiency standpoint. As such, epitaxial element based processing offers several advantages over bulk wafer approaches. As the solar industry has discovered epitaxial element and foil based processing is much more cost effective than wafer based approaches if high resolution lithography is not required. The need exists for cost effective thin nitride epitaxial elements, but also for the ability to process both sides of these epitaxial elements for the formation of waveguides, edge emitters, and symmetric device structures.
While wafers offer significant advantages to high-resolution circuitry as used in microprocessors and memory devices, which require up to 50 masking steps, many applications do not require high-resolution lithography steps. In some cases, processing round wafers is actually a disadvantage or limitation to be overcome. As an example, solar cells are made in ribbon and large area formats. Alternately, liquid crystal displays contain semiconducting active-matrix backplanes and are grown on large glass plates that measure four to 6 feet in dimension. Unlike the semiconductor industry, the thick epitaxial element industry tends to use square substrates, which are more compatible with the printing techniques typically used to form patterned conductors and dielectrics. Square, ribbon and tape based substrates have less edge loss than yields on round wafers. The need exists for non-wafer forms of nitrides to reduce not only costs but to increase material utilization.
In the case of wafer bonded epitaxial elements, stresses created during the original growth process are transferred via the wafer bonding process. Also the resulting structure suffers from poor thermal performance, thermal expansion mismatch which limits operating range. Typically these flexible freestanding epitaxial elements are thin (1 to 3 microns) which can impact packaging processes such as wire bonding to electrical contacts on the device. This is due to the fragile nature of the thin nitride film, requiring reduced bonding forces to prevent cracking of the nitride layer. This lowers yield in final device fabrication. The lower permissible operating temperature range due to the use of secondary substrates used in wafer bonding not only limits device operation but also prevents the use of robust materials such as glass frits and fired contacts that can enhance packaging reliability but require high temperature processing. The need exists for methods and materials which eliminate the need for wafer bonding while still providing handleability and high temperature packaging techniques.
With these prior art methods, nitride epitaxial elements left on their growth substrate must be thin to prevent bowing either at room temperature or at high temperature growth conditions. The low thermal conductivity of sapphire in particular limits device performance and also typically requires wafer thinning processes, which increase costs. Also, the inability to create a vertical structure for non-conductive growth substrates like sapphire limit the ability to form vertical structures or stack nitride epitaxial elements, which can result in new and novel devices. The need exists for an epitaxial element which can be a freestanding nitride growth substrate that is compatible with vertical device designs.
Another prior art technique to form an epitaxial element that is a freestanding nitride substrate is to grow thick nitride layers on non-native growth substrates, followed by dicing and polishing to eliminate the growth substrate. The surface of the thick nitride layer must be touched to remove the growth substrate. This has proven to be a very costly process and by necessity has size limitations. Further, defects introduced via dicing and polishing and the inclusion of stresses due to the dicing processes limit the yield and viability of this approach. In these prior art techniques dual sided processing is impossible or very difficult. However, dual sided processing could offer unique advantages and lead to new types of devices.
In general, the need exists for high quality epitaxial elements which are epi-ready virgin as grown substrates that can be processed, handled, grown upon, and compatible with high temperature packaging and interconnect methods.